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Corm Rot of Giant Swamp Taro (Cyrtosperma Merkusii) Caused by the Burrowing Nematode Radopholus Similis (Nematoda: Pratylenchidae) in the Pacific

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Corm Rot of Giant Swamp Taro (Cyrtosperma Merkusii) Caused by the Burrowing Nematode Radopholus Similis (Nematoda: Pratylenchidae) in the Pacific Nematology, 2005, Vol. 7(4), 631-636 Corm rot of giant swamp taro (Cyrtosperma merkusii) caused by the burrowing nematode Radopholus similis (Nematoda: Pratylenchidae) in the Pacific Vazhaveli K. MURUKESAN 1,EstherVAN DEN BERG 2,LaurensR.TIEDT 3, ∗ Puthiyaparambil C. JOSEKUTTY 4 and Dirk DE WAELE 5, 1 College of Micronesia, Yap State Campus, P.O. Box 1226 Colonia, YAP, FM 96943, Federated States of Micronesia 2 National Collection of Nematodes, Biosystematics Division, Plant Protection Research Institute, Private Bag X134, Queenswood 0121, South Africa 3 Laboratory for Electron Microscopy, North West University, Potchefstroom Campus, Potchefstroom 2520, South Africa 4 Cellular and Molecular Biology Laboratory, School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand 5 Laboratory of Tropical Crop Improvement, Catholic University Leuven (K.U.Leuven), Kasteelpark Arenberg 13, 3001 Leuven, Belgium Received: 28 February 2005; revised: 5 July 2005 Accepted for publication: 6 July 2005 Summary – The association between the burrowing nematode Radopholus similis and giant swamp taro is studied in detail for the first time in corms collected from swampy taro patches in Yap, one of the states of the Federated States of Micronesia. The R. similis population from Yap displays similar variation in morphometrics and morphology as reported in the literature. The rot caused by the nematodes is wet with a loose mass of brown dead tissues and a deep brown necrotic centre housing nematodes inside. Usually, the infected tissues spread a disgusting odour typical of this disease. The dead tissues progress into shallow to deep cavities that advance towards the edible, central portion of the corm giving a perforated appearance on the outside of the otherwise smooth corm. The disease becomes more severe as the age of the plant increases. Nematode-infected plants seldom show any above-ground symptoms. The market quality of the corm is greatly reduced by the nematode damage. The widespread occurrence of the disease and the type of damage R. similis causes to the corms pose a serious threat to giant swamp taro production, food security and the continuation of traditional customs on those islands where R. similis occurs. Keywords – Federated States of Micronesia, plant-parasitic nematode, symptomatology, Yap. Giant swamp taro, Cyrtosperma merkusii (Hassk.) or more and thereby provide food security. Moreover, Schott, is an important crop on many Pacific islands where giant swamp taro also plays a significant role in the it is grown in flooded swamp land for its large edible traditional life of the islanders, especially in Yap where corms. It is an important source of starch, is rich in natural for many centuries it has been considered a cultivated fibres and carbohydrates and is low in fat. In Yap, one plant of status and where a healthy corm has a significant of the states of the Federated States of Micronesia, an value in funerals and traditional ceremonies. Corms from adult consumes about 800 g fresh weight of corm daily plants 10 or more years old are among the most highly (Nakano, 1999). Some of the yellow cultivars are reported prized gifts that a person can give to chiefs during these to be high in carotenoids, making them an ideal local funerals and ceremonies. Also, a number of strong taboos resource to fight vitamin A deficiency among the people and restrictions surround the cultivation of giant swamp of the Pacific islands (Englberger et al., 2003). The crop taro and its preparation for food. is not seasonal and withstands strong winds, the corm In 1987, a corm rot of Cyrtosperma caused by Radopho- having the potential to remain in the ground for 10 years lus similis (Cobb, 1893) Thorne, 1949 was reported from ∗ Corresponding author, e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2005 631 Also available online - www.brill.nl V.K. Murukesan et al. the Pacific islands of Yap, Palau and Guam (Jackson, Evans, 1993). For scanning electron microscopy, speci- 1987). Infected corms had small shallow holes as if they mens were killed and fixed in TAF after which they were had been bored by insects, the holes being 0.5-2 cm in dehydrated in increasing concentrations of amyl acetate diam. and 1-2 cm deep. Beneath this tissue was a brown in pure alcohol and finally in pure amyl acetate. Follow- rot, mostly superficial, but occasionally extending as nar- ing conventional critical point drying and gold/palladium row channels deep into the centre of the corm. Because coating (15 nm), specimens were viewed with a Philips of the rot a considerable amount of decayed tissue had XL30 DX41 stereoscan microscope at 10Kv. For in situ to be pared away before corms became fit for human studies, corm tissue was placed in a concentration of 30% consumption. Infected plants showed little above-ground acetone overnight and then for 30 min each in concentra- symptoms. Large numbers of R. similis were found in the tions of 75%, 95%, and then left in 100% acetone. The corms, but relatively few in the roots (Jackson, 1987). corm tissue was then critical point dried and coated and As R. similis had not previously been reported from Cyr- viewed as described above for the nematode specimens. tosperma and an anaerobic flooded swamp was thought an unlikely habitat for this nematode species, specimens were sent to the Institute of Parasitology in St Albans, Results and discussion England, where the identification was confirmed by tax- onomists. Grandison (1990) reported that in Yap and MEASUREMENTS Palau the incidence of corm rot of giant swamp taro See Table 1 for morphometrics of female and male caused by R. similis was at least 80% and that there were nematodes. indications that the disease had become more important in recent years. In a recent study in Yap (Onjo et al., 2001) MORPHOLOGICAL OBSERVATIONS the validity of R. similis being the causal agent of dry corm rot of giant swamp taro was questioned as the au- The R. similis specimens from Yap compare very well thors failed to extract R. similis from diseased corms. As with various descriptions of this species (Koshy et al., no detailed study of this interesting and important plant- 1991; Ryss & Wouts, 1997; Valette et al., 1998; Elbadri nematode interaction had ever been made, samples were et al., 1999a, b) (Fig. 1). These authors made detailed once again collected in Yap for further study. studies of the morphology and morphometrics of R. In the present study, the identity and the role of R. similis populations from many different countries and host similis in corm rot of giant swamp taro is confirmed. plants. They found a substantial variation in all cuticular Information is given on the morphology of R. similis and morphometrical characters of both females and males. isolated from Cyrtosperma and the symptoms caused by The population from Yap displays similar variation as this nematode species documented. Implications for the recorded by these authors. practical management of R. similis on giant swamp taro are discussed. DISEASE SYMPTOMS The nematode-infected plants seldom show any above- Materials and methods ground symptoms. Giant swamp taro is a perennial crop and the disease becomes more severe as the age of the Infected corms were collected from a knee-deep swam- plant increases. The disease has in the past been named py taro patch at Gagil Municipality, Yap, and brought to ‘dry corm rot’ to distinguish it from soft rots caused by the laboratory in an ice chest. Main and feeder roots were bacteria and fungi and in analogy with dry rots caused by cut off and the corms gently washed to remove the mud Radopholus and Pratylenchus spp. in other root and tuber and soil particles. Small pieces of infected corms together crops, such as yam. Our observations, however, show that with encircling healthy portions were wrapped in tissue the infected necrotic tissues are not dry as the original paper and sent to the Plant Protection Research Institute name of the disease suggests. On the corms from Yap in Pretoria, South Africa, for isolation of the nematodes examined in the present study, the rot is wet with a loose and identification. mass of brown dead tissues and a deep brown necrotic For mounting, the nematodes were transferred to for- centre housing nematodes inside (Fig. 2A, B). Usually, malin-propionic acid-glycerin (FPG) and mounted in an- the infected tissues produce a disgusting odour which is hydrous glycerin (Netscher & Seinhorst, 1969; Hooper & so typical of this disease. Corms are ready for harvest 632 Nematology Corm rot of giant swamp taro Table 1. Morphometrics of adult Radopholus similis from Yap Island. All measurements are in µm and in the form: mean ± standard deviation (range). Female Male n2311 L 604 ± 54 (503-726) 586 ± 54.2 (454-646) a26± 3.4 (21-31.1) 32.8 ± 4.6 (23.5-40) b7.4 ± 0.7 (5.9-8.8) 7.3 ± 0.7 (6.1-8.4) b1 4.5 ± 0.3 (3.8-5) 5.4 ± 0.3 (4.9-5.9) c8.7 ± 0.7 (7.8-10.8) 8 ± 0.6 (6.5-8.4) c1 4.1 ± 0.4 (3.4-4.6) 5.2 ± 0.2 (4.9-5.7) o20.1 ± 3.6 (13-26.7) – V57± 1.6 (53-59.5) – DGO 3 ± 0.5 (2-4) – OV1 23 ± 5.1 (19-43) – OV2 21 ± 2.9 (17-29) – Stylet length 16.5 ± 0.5 (15-17) – Metenchium length 8 ± 0.4 (8-8.5) – Telenchium length 8.5 ± 0.5 (8-9) – Stylet knob height 2 ± 0.3 (1.5-2.5) – Stylet knob width 3.5 ± 0.3 (3-4.5) – Median bulb length 12.5 ± 1.1 (11-14) 11 ± 1.4 (8-13) Median bulb diam.
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